The present invention relates to a method of forming a cylindrical portion of a sheet metal member, the cylindrical portion to being a pulley having a belt winding-hanging portion for rollingly hanging a flat belt, a V-shaped belt, a poly-V-belt or the like, and having a gear provided with teeth on an inner surface or an outer surface of the cylindrical portion. Specifically, the present invention relates to a method of pressing the cylindrical portion of the sheet metal member having a center hole or a boss portion, which requires to be a high degree of accuracy as to its shape, or its measurements, disposed on a center of a circular base plate, in a radial direction of the base plate by means of a forming roller, thus shaping it into a predetermined shape or length.
When forming a cylindrical portion of a sheet metal member having a center hole, wherein a cylindrical boss portion protrudes from a center of a circular base plate whose outer periphery is connectedly provided with a cylindrical portion, and the sheet metal material having the center hole is formed by the boss portion it is known to hold the sheet metal material is between rotating dies. A rod portion arranged at the rotating die is fitted into the center hole, so as to align a rotating axis center of the rotating die with the center of the base plate, thereby rotating the sheet metal material with the rotating dies, while pressing the cylindrical portion in a radial direction of the base plate by means of a forming roller leading to forming the cylindrical portion. The influence of pressure by means of the forming roller shifts the base plate slightly radially, a circumferential part of the boss portion is pressed strongly against the rod portion, whereby high accuracy as to shape and measurements of the sheet metal member as a final product obtained by forming the cylindrical portion cannot be achieved owing to distorted transformation of the center hole formed by the boss portion. Accordingly, conventionally, after forming the cylindrical portion, the center hole has been cut again to maintain the high accuracy thereof. Similarly, when forming a cylindrical portion by use of the sheet metal material wherein the center hole is formed by a circular opening disposed on the center of the base plate, the same state is possibly caused.
It is an object of the present invention to provide a method of forming a cylindrical portion of a sheet metal member having a center hole, wherein, in the case of forming a cylindrical portion according to the above mentioned method, there is no fear of deteriorating the accuracy of the center hole arranged on the center of the base plate owing to the influence of the pressure of the forming roller.
In order to achieve the above noted object, a method of forming a cylindrical portion of a sheet metal member having a center hole, according to the present invention, comprises the steps of:
holding a circular base plate of a sheet metal material having a center hole disposed on the base plate, in an interposed state, between rotating dies,
fitting a rod portion concentrically with each rotating axis center, disposed on the rotating dies, into the center hole of the sheet metal material,
making the sheet metal material rotate with the rotating dies, and
pressing a cylindrical portion connectedly disposed on an outer periphery of the base plate in a radially inward direction of the base plate by means of a forming roller, so as to form the cylindrical portion,
wherein each engagement disposed on portions where the rotating dies are overlapped with the base plate, is engaged with each engaged portion disposed on the base plate, so as to prevent the base plate from moving radially.
The center hole includes a hole formed by a circular opening disposed on the center of the base plate and a hole formed by a cylindrical boss portion, connecting to the center of the base plate.
According to the above method of forming the cylindrical portion, a force presses the cylindrical portion of the sheet metal material in a radial direction thereof by means of the forming roller, and the cylindrical portion is supported by the engagement of the rotating die engaged with the engaged portion via the engaged portion of the base plate, thereby preventing the base plate from moving slightly. Consequently, during forming, an influence of pressure by the forming roller prevents a state wherein a hole circumference of the center hole of the sheet metal material fitted by the rod portion on the side of the rotating die, is strongly pressed against the rod portion to transform it distortedly, so that accuracy in the shape and measurements of the center hole prior to forming the cylindrical portion is maintained even after it is formed.
Preferably, according to the present invention, as the rotating dies, a first rotating die and a second rotating die for holding the base plate are employed, and the engagements disposed on both the first and second rotating dies, are respectively engaged with the engaged portions respectively disposed on a front surface and a back surface of the base plate, so as to prevent the base plate from radially moving.
Thus, the force in pressing the cylindrical portion of the sheet metal material in a radial direction by means of the forming roller is supported by each engagement of the first rotating die and the second rotating die engaged with each engaged portion via each engaged portion disposed on the front and back surfaces of the base plate. Accordingly, this ensures an effect wherein the accuracy in the shape and measurements of the center hole of the cylindrical portion prior to forming the cylindrical portion mentioned above, is maintained even after forming the cylindrical portion.
In the present invention, preferably, the engagements and the engaged portions are circularly annular with the result that the forming roller presses the cylindrical portion of the sheet metal material rotating with the rotating dies in a radial direction, thereby always exhibiting an effect of supporting the force by the cooperation of the engaged surfaces of the engaged portions.